rtl8xxxu: Rename rtl8723a_enable_rf() to rtl8xxxu_gen1_enable_rf()

All gen1 parts use the same enable_rf() function, so rename it to
reflect this.

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