usb: Fix qcserial memory leak on rmmod

qcprobe function allocates serial->private but this is never freed, this
patch adds a new function qc_release() which frees serial->private, after
calling usb_wwan_release

Signed-off-by: Steven Hardy <shardy@redhat.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>