rtlwifi: remove duplicate code

Remove and refactor some code in order to avoid having identical code
for different branches.

Notice that the logic has been there since 2014.

Addresses-Coverity-ID: 1426199 ("Identical code for different branches")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>