usb: musb: sysfs mode updates

Fix three omissions in the "mode" sysfs attribute support:
  (a) inability to report errors;
  (b) no DaVinci support ... just report an error;
  (c) for omap2430, accepting unsupportable values

The 2430 stuff is still odd....

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>