[media] em28xx-input: i2c IR decoders: improve i2c_client handling

Instead of using a temporary stack allocated i2c_client in em28xx_i2c_ir_handle_key(),
allocate/free the i2c_client at module init/uninit and hook it into struct em28xx_IR
(if the device has an i2c IR decoder).
This reduces the frame size of function em28xx_i2c_ir_handle_key() and speeds
it up a bit.
Also make sure that all fields of struct i2c_client are initialized properly.

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