usb: musb: ignore spurious SESSREQ interrupts

This will ignore any SESSREQ interrupt if musb is B state.
Charger detection may cause spurious SESSREQ interrupts.

Signed-off-by: Heikki Krogerus <ext-heikki.krogerus@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>