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.

Change-Id: Icf3430e355acc2c33dc37c6a38a18c47de03a2cf
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>