staging: r8188eu: use ieee80211 helper to read "more data"

Use the ieee80211 helper to read the "more data" bit. The driver-specific
macro GetMData can be removed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220227164147.1168847-7-martin@kaiser.cx


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