[media] rtl28xxu: merge chip type specific all callbacks

Merge all chip type specific prefixed (rtl2831u_ and rtl2832u_)
callback to top level callback prefixed as rtl28xxu_.

rtl2831u_foo() => rtl28xxu_foo()
rtl2832u_foo() => rtl28xxu_foo()

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>