rtl8xxxu: add enumeration for channel bandwidth

There's a data field in H2C and C2H commands which is used to
carry channel bandwidth information. Add enumeration to make it
more descriptive in code.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200320063833.1058-2-chiu@endlessm.com