rtl8xxxu: Rename rtl8723au_update_rate_mask() to rtl8xxxu_update_rate_mask()

All currently supported gen1 parts use the same function for updating
the rate mask, so rename it to reflect this.

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