rtlwifi: Fix regression caused by commit d86e6476

In commit d86e6476 ("rtlwifi: rtl818x: constify local structures"),
the configuration struct for most of the drivers was changed to be
constant. The problem is that five of the modified drivers need to be
able to update the firmware name based on the exact model of the card.
As the file names were stored in one of the members of that struct,
these drivers would fail with a kernel BUG splat when they tried to
update the firmware name.

Rather than reverting the previous commit, I used a suggestion by
Johannes Berg and made the firmware file name pointers be local to
the routines that update the software variables.

The configuration struct of rtl8192cu, which was not touched in the
previous patch, is now constantfied.

Fixes: d86e6476

 ("rtlwifi: rtl818x: constify local structures")
Suggested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> # 4.8
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>