scsi: mptconfig: fix an assert expression

When LSI SAS1068 Host Bus emulator builds configuration page
headers, mptsas_config_pack() should assert that the size
fits in a byte.  However, the size is expressed in 32-bit
units, so up to 1020 bytes fit.  The assertion was only
allowing replies up to 252 bytes, so fix it.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <1472645167-30765-2-git-send-email-ppandit@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>