Input: tca8418 - remove set but not used variable 'max_keys'

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

drivers/input/keyboard/tca8418_keypad.c: In function tca8418_keypad_probe:
drivers/input/keyboard/tca8418_keypad.c:269:24: warning: variable max_keys set but not used [-Wunused-but-set-variable]

It's not used since commit 16ff7cb1 ("Input:
tca8418-keypad - switch to using managed resources")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>