[LSM6DSRX] Improved fifo reading by performing a single spi read.

src/shared/sensors/LSM6DSRX/LSM6DSRX.cpp

src/shared/sensors/LSM6DSRX/LSM6DSRX.h

src/shared/sensors/LSM6DSRX/LSM6DSRXDefs.h

src/tests/sensors/test-lsm6dsrx.cpp