spi/bcm63xx: Remove the unused function bcm_spi_readw()

spi/bcm63xx: Remove the unused function bcm_spi_readw()

The function bcm_spi_readw is defined in the spi-bcm63xx.c file, but
not called elsewhere, so remove this unused function.

drivers/spi/spi-bcm63xx.c:160:19: warning: unused function 'bcm_spi_readw'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4242
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230228023243.118429-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Mark Brown <broonie@kernel.org>