megasas: use unsigned type for positive numeric fields

Use unsigned type for the MegasasState fields which hold positive
numeric values.

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20200513192540.1583887-4-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>