usb: musb: omap2+: save and restore OTG_INTERFSEL

we need to save and restore OTG_INTERFSEL register
else we will be unable to function on resume after
OFF mode.

Reported-by: Devaraj Rangasamy <dev@ti.com>
Signed-off-by: Hema HK <hemahk@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>