staging: r8188eu: use ieee80211 helper to read the pwr bit

Use the ieee80211 helper to read the power management bit.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220327180944.712545-3-martin@kaiser.cx


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