rtl8xxxu: Implement rtl8192e_enable_rf()

This implements an 8192eu specific enable_rf() function. The 8192eu is
not a combo device, so no need for doing the BT specific bits needed
by the 8723bu.

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