Input: rc-keymap - return KEY_RESERVED for unknown mappings

Do not respond with -EINVAL to EVIOCGKEYCODE for not-yet-mapped
scancodes, but rather return KEY_RESERVED.

This fixes breakage with Ubuntu's input-kbd utility that stopped
returning full keymaps for remote controls.

Tested-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Mark Lord <kernel@teksavvy.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>