s390/ebcdic: Fix length check in codepage_convert()

The current code compares whether the nr argument is less or equal to
zero. As nr is of type unsigned long, this isn't correct. Fix this by just
testing for zero. This is also reported by checkpatch:

unsignedLessThanZero: Checking if unsigned expression 'nr--' is less
than zero.

Reported-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>