usb: musb: omap2430: make sure clocks are enabled when running mailbox

on early initialization we could fall into
a situation where the mailbox is called before
MUSB's clocks are running, in order to avoid
that, make sure mailbox is always wrapped with
pm_runtime calls.

Reported-by: Stefan Roese <sr@denx.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>