usb: pd: Fix unbalanced vconn regulator disable

Make sure to only call regulator_disable() on VCONN only if
it was previously enabled, else it could result in an
unbalanced disable state. Subsequent enables would then be
no-ops and the regulator would not actually turn on.

Change-Id: I67a6ad6d2bc036a63714403e2eed8c0577ef0246
Signed-off-by: Jack Pham <jackp@codeaurora.org>