mwifiex: add missing check for PCIe8997 chipset

This patch ensures mwifiex_pcie_txbd_empty() does take care
of 8997 chipset.

Fixes: 6d85ef00 ("mwifiex: add support for 8997 chipset")
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>