staging: rtl8723au: Reorganize a few functions to remove indentation

Prior to this commit, functions rtw_tkip_encrypt23a and rtw_tkip_decrypt23a had
large if blocks which contained the majority of the logic in the functions.

Rework these functions so that if the negated version of the aforementioned if
blocks' conditions are true, we return from the function with _FAIL, as
expected by the calling code.

This lets us remove two levels of indentation from the functions in
question, making them more readable.

Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>