prism54: off by one BUG_ON() test

This code was supposed to trigger a BUG() if we truncate the output but
it's off by one so it allows one character to be truncated.  Really
drivers shouldn't call BUG_ON() and especially for something minor like
this so I've changed it to a WARN_ON().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>