staging:iio:gyro:adxrs450 make more use of spi_read and spi_write.

This needs confirmation that the devices is happy if another part is
talked to in between the request for a register and the read back.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>