HAL_Linux: update SPI speeds for Navigator

As @jaxxzer found out, if SCL goes under 1MHz, the MMC sometimes
glitches out, freezing.

libraries/AP_HAL_Linux/SPIDevice.cpp

@@ -80,8 +80,8 @@ SPIDesc SPIDeviceManager::_device[] = {
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_NAVIGATOR
 SPIDesc SPIDeviceManager::_device[] = {
     SPIDesc("led",        0, 0, SPI_MODE_0, 8, SPI_CS_KERNEL,  6*MHZ, 6*MHZ),
-    SPIDesc("icm20602",   1, 2, SPI_MODE_0, 8, SPI_CS_KERNEL,  1*MHZ, 10*MHZ),
-    SPIDesc("mmc5983",    1, 1, SPI_MODE_0, 8, SPI_CS_KERNEL,  1*MHZ, 10*MHZ),
+    SPIDesc("icm20602",   1, 2, SPI_MODE_0, 8, SPI_CS_KERNEL,  4*MHZ, 10*MHZ),
+    SPIDesc("mmc5983",    1, 1, SPI_MODE_0, 8, SPI_CS_KERNEL,  4*MHZ, 10*MHZ),
 };
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_NAVIO || CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_NAVIO2
 SPIDesc SPIDeviceManager::_device[] = {