pc_sysfw: Fix ISA BIOS init for ridiculously big flash

pc_isa_bios_init() suffers integer overflow for flash larger than
INT_MAX.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1375276272-15988-9-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <anthony@codemonkey.ws>
