Input: palmas-pwrbutton - fix typo in the license string

Follow the license string convention indicated in include/linux/module.h
(don't capitalize v).

This fixes following randconfig warning:
FATAL: modpost: GPL-incompatible module palmas-pwrbutton.ko uses GPL-only symbol 'platform_driver_unregister'

Fixes: adff5962

 ("Input: introduce palmas-pwrbutton")
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>