p54spi: cosmetic fixes: use even byte count in SPI write; drop unused interrupt status read

When SPI write of odd length is requested, p54spi_write splits it
into two parts: one for all data, except the last byte, and one
for last byte and padding byte. Unfortunately, the length of
first part is not amended. It works because all meaningful bytes
have proper value and the last byte of odd length SPI write
transaction is ignored.

p54spi_work has dummy HOST_INTERRUPTS register read at the end.
Drop it, as its result is not used and it has no side effects.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>