Revert "ADC: only read channels that are actually being used"

This reverts commit 95e6bb26f4772ca8c7faa62215de6f078d3df3c7.

ADC will be disabled on APM2

libraries/AP_ADC/AP_ADC_ADS7844.cpp

@@ -63,11 +63,6 @@ static volatile uint32_t _sum[8];
 // how many values we've accumulated since last read
 static volatile uint16_t _count[8];
 
-// a mask of what channels are actually being read. If a channel has
-// never been read, then don't bother gathering it. That saves us a
-// lot of cycles in the timer call
-static uint8_t enable_mask;
-
 static uint32_t last_ch6_micros;
 
 // TCNT2 values for various interrupt rates,
@@ -92,35 +87,15 @@ static inline unsigned char ADC_SPI_transfer(unsigned char data)
 void AP_ADC_ADS7844::read(void)
 {
 	uint8_t ch;
-	unsigned char enable_cmd[9];
-	uint8_t num_enabled = 0;
-
-	if (enable_mask == 0) {
-		// no channels to read
-		return;
-	}
-
-	for (ch = 0; ch < 8; ch++) {
-		if (enable_mask & (1<<ch)) {
-			enable_cmd[num_enabled++] = adc_cmd[ch];
-		}
-	}
-	enable_cmd[num_enabled] = 0;
-
-	num_enabled = 0;
 
 	bit_clear(PORTC, 4);							// Enable Chip Select (PIN PC4)
-	ADC_SPI_transfer(enable_cmd[0]);					// Command to read the first channel
+	ADC_SPI_transfer(adc_cmd[0]);						// Command to read the first channel
 
 	for (ch = 0; ch < 8; ch++) {
 		uint16_t v;
 
-		if ((enable_mask & (1<<ch)) == 0) {
-			continue;
-		}
-		
 		v = ADC_SPI_transfer(0) << 8;	         // Read first byte
-		v |= ADC_SPI_transfer(enable_cmd[++num_enabled]);  // Read second byte and send next command
+		v |= ADC_SPI_transfer(adc_cmd[ch + 1]);  // Read second byte and send next command
 
 		if (v & 0x8007) {
 			// this is a 12-bit ADC, shifted by 3 bits.
@@ -193,8 +168,6 @@ float AP_ADC_ADS7844::Ch(uint8_t ch_num)
 	uint16_t count;
 	uint32_t sum;
 
-	enable_mask |= (1<<ch_num);
-
 	// ensure we have at least one value
 	while (_count[ch_num] == 0)  /* noop */ ;
 
@@ -220,10 +193,6 @@ uint32_t AP_ADC_ADS7844::Ch6(const uint8_t *channel_numbers, uint16_t *result)
 	uint32_t sum[6];
 	uint8_t i;
 
-	for (i=0; i<6; i++) {
-		enable_mask |= (1<<channel_numbers[i]);
-	}
-
 	// ensure we have at least one value
 	for (i=0; i<6; i++) {
 		while (_count[channel_numbers[i]] == 0) /* noop */;