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

This partially reverts commit cb4496876f03631eff913b3c608c964d48d61eb9. There is
no apparent reason why we should split a transaction which consists out of
multiple transfers into multiple transactions each having one transfer. While
this works it introduces a bunch of unnecessary context switch and additional
setup costs.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>