usb: ehci-msm2: Make vbus-supply optional

Some configurations may not need a regulator for enabling
and disabling VBUS, such as when the port is hardwired to
an always-on power rail, or in cases where a peripheral is
externally powered. Relax the requirement that the
"vbus-supply" regulator be always provided by ignoring
error return values from the call to regulator_get().

Change-Id: Ib353f313ef3efa8cd135db73803aa932a0994cb6
Signed-off-by: Jack Pham <jackp@codeaurora.org>