Input: nspire-keypad - add missing clk_disable_unprepare() on error path

Add the missing clk_disable_unprepare() before return
from nspire_keypad_open() in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>