rtl8xxxu: Rename rtl8723a_disable_rf() to rtl8xxxu_gen1_disable_rf()

All currently supported gen1 parts use the same disable_rf() routine,
so rename the function to reflect that.

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