staging: wfx: fix an error handling in wfx_init_common()

One error handler of wfx_init_common() return without calling
ieee80211_free_hw(hw), which may result in memory leak. And I add
one err label to unify the error handler, which is useful for the
subsequent changes.

Suggested-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Link: https://lore.kernel.org/r/tencent_24A24A3EFF61206ECCC4B94B1C5C1454E108@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>