p54spi: remove dead code and definitions

This patch removes some dead code:
p54spi.c:115: warning: ‘p54spi_read16’ defined but not used

and while we're at it, p54spi_registers_array is purged as well.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>