usb: musb: fix bug in musb_schedule

This bug was introduced recently. Fix it before bigger
problems appear.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>