spi: spi-au1550: replace ternary operator with min()

Fix the following coccicheck warnings:

drivers/spi/spi-au1550.c:408:21-22: WARNING opportunity for min()
drivers/spi/spi-au1550.c:542:21-22: WARNING opportunity for min()

min() macro is defined in include/linux/minmax.h. It avoids multiple
evaluations of the arguments when non-constant and performs strict
type-checking.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20220513130333.58379-1-guozhengkui@vivo.com
Signed-off-by: Mark Brown <broonie@kernel.org>