media: rc: imon: Allow iMON RC protocol for ffdc 7e device

Allow selecting the IR protocol, MCE or iMON, for a device that
identifies as follows (with config id 0x7e):

15c2:ffdc SoundGraph Inc. iMON PAD Remote Controller

As the driver is structured to default to iMON when both RC
protocols are supported, existing users of this device (using MCE
protocol) will need to manually switch to MCE (RC-6) protocol from
userspace (with ir-keytable, sysfs).

Signed-off-by: Darius Rad <alpha@area49.net>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>