mwifiex: fix adapter pointer dereference issue

It has introduced by recent commit 6b41f941: "mwifiex:
handle driver initialization error paths" which adds error
path handling for mwifiex_fw_dpc().

release_firmware(adapter->*) is called for success as well
as failure paths. In failure paths, adapter is already freed
at this point.

The issue is fixed by moving mwifiex_free_adapter() call.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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>