staging: unisys: visornic: convert BUG_ON to dev_err() message

The caller of visor_copy_fragsinfo_from_skb() is prepared to log a
dev_err() message if it fails (by returning a negative value), so we now
just fail in the one occasion where visor_copy_fragsinfo_from_skb() was
doing a BUG_ON.

There was also a problem before where visor_copy_fragsinfo_from_skb() was
returning a negative to indicate error, yet it was declared to return an
unsigned value!  So obviously that needed correcting too.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>