mwifiex: do not wait on semaphore during card removal

Host hang is observed if card is removed before firmware download
gets completed. In this case, firmware will be failed to download and
adapter structure gets freed.

In other thread, mwifiex_remove_card() waits on semaphore until the
firmware download fails. This wait is not necessary and may result in
invalid adapter access.

This patch uses down_trylock to return immediately so that hang issue
won't occur.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>