mwifiex: lock main process till reinitialization of vif is over

A crash was detected while changing virtual interface type is in
progress. This was tracked to race condition in accessing bss_priority
table while change is in progress. This patch ensures that main_process
and rx_process works are locked while we change virtual interface.

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>