scsi: mptconfig: fix misuse of MPTSAS_CONFIG_PACK

These issues cause respectively a QEMU crash and a leak of 2 bytes of
stack.  They were discovered by VictorV of 360 Marvel Team.

Reported-by: Tom Victor <i-tangtianwen@360.cm>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>