usb: chipidea: host: Only disable the vbus regulator if it is not NULL

Commit 40ed51a4 (usb: chipidea: host: add vbus regulator
control) introduced a smatch complaint because regulator_disable() is called
without checking whether ci->platdata->reg_vbus is not NULL.

Fix this by adding the check.

This patch is needed for 3.12 stable

Cc: stable <stable@vger.kernel.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>