usb: phy: mxs: declare variable with initialized value

Initialize vbus_value to be 0 since it's possible not to assign
any value before judgement.

Acked-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>