HAL_PX4: fixed dirty_mask calculation in FRAM storage

this could lead to a number of bytes on 512 byte boundaries not being
written when changed in ram, so they would revert on next boot

libraries/AP_HAL_PX4/Storage.cpp

@@ -176,12 +176,12 @@ void PX4Storage::_storage_open(void)
  */
 void PX4Storage::_mark_dirty(uint16_t loc, uint16_t length)
 {
-	uint16_t end = loc + length;
-	while (loc < end) {
-		uint8_t line = (loc >> PX4_STORAGE_LINE_SHIFT);
-		_dirty_mask |= 1 << line;
-		loc += PX4_STORAGE_LINE_SIZE;
-	}
+    uint16_t end = loc + length;
+    for (uint8_t line=loc>>PX4_STORAGE_LINE_SHIFT;
+         line <= end>>PX4_STORAGE_LINE_SHIFT;
+         line++) {
+        _dirty_mask |= 1U << line;
+    }
 }
 
 void PX4Storage::read_block(void *dst, uint16_t loc, size_t n)