rtl8xxxu: Fix rtl8723bu driver reload issue

The generic disable_rf() function clears bits 22 and 23 in
REG_RX_WAIT_CCA, however we did not re-enable them again in
rtl8723b_enable_rf()

This resolves the problem for me with 8723bu devices not working again
after reloading the driver.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>