staging: r8188eu: merge process_addba_req into OnAction_back

staging: r8188eu: merge process_addba_req into OnAction_back

The process_addba_req function is called only by OnAction_back. Merge the
two functions.

OnAction_back has already checked that psta != NULL, we can drop the check
in process_addba_req before merging the functions. We can also simplify
the parsing of the tid by using struct ieee80211.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220514164740.282552-4-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>