mwifiex: illegal assignment

Variable adapter is incorrectly initialized.

Fixes: bf00dc22 ("mwifiex: AMSDU Rx frame handling in AP mode")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>