usb: musb: restore INDEX register in resume path

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

Cc: stable@kernel.org
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>