mwifiex: implement extended scan feature

In extended scan, host gets scan results through one or
multiple events instead of scan command response. Host will
send next scan command when all the events are received.

Legacy scan sometimes truncates scan results in a noisy
environment due to buffer length limitation. This issue
is addressed in extended scan.

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>