[media] em28xx: get rid of the dependency on module ir-kbd-i2c

We already have the key polling functions and the polling infrastructure in
em28xx-input, so we can easily get rid of the dependency on module ir-kbd-i2c.
For maximum safety, do not touch the key reporting mechanism for those devices.
Code size could be improved further but would have minor peformance impacts.
Tested with device "Terratec Cinergy 200 USB" (EM2800_BOARD_TERRATEC_CINERGY_200)

[mchehab@redhat.com: Fix two checkpatch.pl warnings:
 ERROR: "foo * bar" should be "foo *bar" (line 465)
 WARNING: kfree(NULL) is safe this check is probably not required (line 725)]

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>