staging: r8188eu: exit straight away if we have no pspoll frame

validate_recv_ctrl_frame wraps nearly all of its code into a large
if (pspoll) { ... } clause.

Revert this condition and exit if the incoming frame is not a pspoll
frame.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220403164526.357371-4-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>