Revert "mwifiex: handle race during mwifiex_usb_disconnect"

This reverts commit b817047a.

We have a better fix for this issue, which will be sent on top
of this revert.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>