gpio: line-mux: remove bits already handled by GPIO core

gpio: line-mux: remove bits already handled by GPIO core

GPIO core already handles checking the offset against the number of
GPIOs as well as missing any of the GPIO chip callbacks. Remove the
unnecessary bits.

Also, the offset check was off-by-one as reported by Dan.

Fixes: 2b03d9a40cd1 ("gpio: add gpio-line-mux driver")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/aV4b6GAGz1zyf8Xy@stanley.mountain/
Tested-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Reviewed-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://lore.kernel.org/r/20260107085833.17338-1-bartosz.golaszewski@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>