USB: fix port probing and removal in garmin_gps

This patch (as1615) fixes a bug in the Garmin USB serial driver.  It
uses attach, disconnect, and release routines to carry out actions
that should be handled by port_probe and port_remove routines, because
they access port-specific data.

The bug causes an oops when the device in unplugged, because the
private data for each port structure now gets erased when the port is
unbound from the driver, resulting in a null-pointer dereference.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported--by: Markus Schauler <mschauler@gmail.com>
Tested-by: Markus Schauler <mschauler@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>