brcmfmac: free net device when registration fails

When registration fails the net device is no longer needed. Free
the net device and remove reference to private data from the
driver.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>