usb: musb: omap2+: fix context api's

RxFifoSz, TxFifoSz, RxFifoAddr, TxFifoAddr
are all indexed registers.

So before doing a context save or restore, INDEX register
should be set, then only one gets to the right register offset.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>