brcmsmac: NULL dereferences in brcms_c_detach_mfree()

The brcms_c_attach_malloc() function can call this with a NULL
"wlc->corestate" or "wlc->hw".

Also I threw in a bonus cleanup by deleting an obvious comment and a
no-op NULL assignment.  :)

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>