s390/qdio: replace shift loop by ilog2

account_sbals is called by get_inbound_buffer_frontier and
get_outbound_buffer_frontier with 'count' value > 0 so we can safely
convert shift loop to ilog2.

Cc: Joe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>