usb: musb: only restore devctl when session was set in backup

If the session bit was not set in the backup of devctl register,
restoring devctl would clear the session bit. Therefor, only restore
devctl register when the session bit was set in the backup.

This solves the device enumeration failure in otg mode exposed by commit
56f487c7 (PM / Runtime: Update last_busy in rpm_resume).

Cc: <stable@vger.kernel.org> # v4.2+
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_core.c

@@ -2418,7 +2418,8 @@ static void musb_restore_context(struct musb *musb)
 	musb_writew(musb_base, MUSB_INTRTXE, musb->intrtxe);
 	musb_writew(musb_base, MUSB_INTRRXE, musb->intrrxe);
 	musb_writeb(musb_base, MUSB_INTRUSBE, musb->context.intrusbe);
-	musb_writeb(musb_base, MUSB_DEVCTL, musb->context.devctl);
+	if (musb->context.devctl & MUSB_DEVCTL_SESSION)
+		musb_writeb(musb_base, MUSB_DEVCTL, musb->context.devctl);
 
 	for (i = 0; i < musb->config->num_eps; ++i) {
 		struct musb_hw_ep	*hw_ep;