usb: musb: omap2430: fix omap_musb_mailbox glue check again

Commit 80ab72e1

 (usb: musb: omap2430: fix the readiness check
in omap_musb_mailbox) made the check incorrect, as we will lose the
glue/link status during the normal built-in probe order (twl4030_usb is
probed after musb omap2430, but before musb core is ready).

As a result, if you boot with USB cable on and load g_ether, the
connection does not work as the code thinks the cable is off and the
phy gets powered down immediately. This is a major regression in 3.9-rc1.

So the proper check should be: exit if _glue is NULL, but if it's
initialized we memorize the status, and then check if the musb core
is ready.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>