s390/ebcdic: Fix length decrement in codepage_convert()

The inline assembly uses the ahi instruction to decrement and test
whether more than 256 bytes are left for conversion. But the nr
variable passed is of type unsigned long. Therefore use aghi.

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