Input: applespi - remove set but not used variables 'sts'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/input/keyboard/applespi.c: In function applespi_set_bl_level:
drivers/input/keyboard/applespi.c:902:6: warning: variable sts set but not used [-Wunused-but-set-variable]

Fixes: b426ac0452093d ("Input: add Apple SPI keyboard and trackpad driver")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>