USB: mos7840: fix port-device leak in error path

commit 3eb55cc4 upstream.

The driver set the usb-serial port pointers to NULL on errors in attach,
effectively preventing usb-serial core from decrementing the port ref
counters and releasing the port devices and associated data.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>