wil6210: minimize the time that mem_lock is held

mem_lock is taken for the entire wil_reset().
Optimize this by taking mem_lock just before device is
being reset and release the lock after FW download.

Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>