drv_hrt: added hrt_call_init() and hrt_call_delay() APIs

hrt_call_init() can be used to initialise (zero) a hrt_call structure
to ensure safe usage. The hrt_call_every() interface calls this
automatically.

hrt_call_delay() can be used to delay a current callout by the given
number of microseconds
px4fmu2: enable SPI sensor DRDY pins
lsm303d: use DRDY pins to automatically reschedule measurements

this prevents double reads of sensor data, and missing samples from
the accel
ms5611: change bus speed to 5MHz

this gives 5MHz SPI bus speed (by asking for 6MHz due to timer
granularity).

Tests with a logic analyser show that the ms5611 is actually more
reliable at 5MHz than lower speeds
ms5611: enabled debug()
ms5611: removed unused variable
ms5611: give cleaner SPI traces

this makes logic traces cleaner by zeroing extra bytes written