mwifiex: scan delay timer cleanup in unload path

Return from scan delay timer routine if surprise_removed flag
is true. Also, cancel the timer in unload path.

This fixes a crash when scan delay timer accesses structures
that have been freed already.

Tested with "iwlist mlan0 scan & sleep 1; rmmod mwifiex_sdio"

Reported-by: Daniel Drake <dsd@laptop.org>
Tested-by: Daniel Drake <dsd@laptop.org>
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>