[media] em28xx: IR RC: move assignment of get_key functions from *_change_protocol() functions to em28xx_ir_init()

The get_key functions are independent from the selected protocol, so assign
them once only at device initialization.

[mchehab@redhat.com: fix a merge conflict]
Signed-off-by: NFrank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>