[media] em28xx: input: use common work_struct callback function for IR RC key polling

Remove em28xx_i2c_ir_work() and check the device type in the common callback
function em28xx_ir_work() instead. Simplifies em28xx_ir_start().
Reduces the code size with a minor performance drawback.

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