mwifiex: handle driver initialization error paths

mwifiex_fw_dpc() asynchronously takes care of firmware download
and initialization. Currently the error paths in mwifiex_fw_dpc()
are not handled. So if wrong firmware is downloaded, required
cleanup work is not performed. memory is leaked and workqueue
remains unterminated in this case.

mwifiex_terminate_workqueue() is moved to avoid forward
declaration.

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>