mwifiex: fix regression issue for usb interface

PATCH "mwifiex: scan delay timer cleanup in unload path" adds code
to cancel scan delay timer in unload path. It causes a regression
for USB interface.

USB8797 card gets enumerated twice. First enumeration is for
firmware download and second enumeration expects firmware
initialization.

It was observed that we are trying del_timer_sync() without setting
up the timer when remove handler is called after first enumeration.

This patch moves setup_timer() call to appropriate place so that
timer is setup for both the enumerations.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>