usb: musb: restore INDEX register in resume path

BugLink: http://bugs.launchpad.net/bugs/823296

commit 3c5fec75e121b21a2eb35e5a6b44291509abba6f upstream.

Restoring the missing INDEX register value in musb_restore_context().
Without this suspend resume functionality is broken with offmode
enabled.

Acked-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>