usb: musb: be careful when using different fifo_modes

if we have more endpoints configured than
enabled on fifo_mode, then we need to be
careful on save/restore context operations,
otherwise we will try to access uninitialized
__iomem pointer.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>