staging: r8188eu: use ieee80211 helper to check for pspoll

Use the ieee80211 helper to check if our incoming ctrl frame is a
pspoll frame.

We can drop the initial ctrl frame check as ieee80211_is_pspoll
checks for a control frame with subtype pspoll.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220403164526.357371-3-martin@kaiser.cx


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