mwifiex: schedule main workqueue for transmitting bridge packets

Bridge packets are enqueued to wmm tx queue, but will not be sent
until main workqeue is scheduled for new interrupt or other
reason. This adds unnecessary delay during traffic.

We will schedule main workqueue when bridge packet is queued.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>