V4L/DVB (12598): dvb-usb: store rc5 custom and data at the same field

In order to be closer to V4L IR implementation, it is needed to replace
	u8 custom, data
to:
	u16 scan

This allows using non-rc5 mappings, like the 8 bit keycodes used on older
protocols. It will also allow future implementations of rc6 keystrokes
when needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>