V4L/DVB (13599): radio-si470x: move some file operations to common file

The read and poll file operations of the si470x usb driver can be used
also equally on the si470x i2c driver, so they go to the common file.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>