[hermon] Work around missing mport support in current BOFM implementations

[hermon] Work around missing mport support in current BOFM implementations

Current BOFM versions are unable to create entries with mport>1, which
means that only the port 1 MAC address can be explicitly specified.

Work around this by using the provided MAC address as a base address
for all subsequent ports.  For example, if BOFM assigns the address

   00:1A:64:76:00:09 for port 1

then we will assign the addresses

   00:1A:64:76:00:09 for port 1
   00:1A:64:76:00:0a for port 2

Future BOFM versions that may correctly support mport will work with
this scheme without modification provided that the BOFM entries are
created in increasing order of mport.  Since BOFM tools tend to
generate entries in increasing order (of slot, port, etc), this is not
an unreasonable compromise.

Signed-off-by: Michael Brown <mcb30@ipxe.org>