usb: musb: omap: fix the error check for pm_runtime_get_sync

pm_runtime_get_sync returns a signed integer. In case of errors
it returns a negative value. This patch fixes the error check
by making it signed instead of unsigned thus preventing register
access if get_sync_fails. Also passes the error cause to the
debug message.

Cc: stable@vger.kernel.org
Cc:  Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>