staging: r8188eu: remove HW_VAR_AMPDU_FACTOR from SetHwReg8188EU()

Remove the case HW_VAR_AMPDU_FACTOR from SetHwReg8188EU() and move
the functionality to a static function in rtw_wlan_util.c. This is
part of the ongoing effort to get rid of SetHwReg8188EU().

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220701210753.8210-1-straube.linux@gmail.com


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