rtl8xxxu: Use REG_RFE_CTRL_ANTA_SRC rather than hard coded value

Another case where we should use the register name rather than the
hard coded value when accessing it.

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