rtl8xxxu: rename rtl8723a_channel_group() to rtl8xxxu_gen1_channel_to_group()

This function is generic for most (if not all) gen1 parts, so rename
it to reflect this.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>