[fc] Do not use the command reference number in FCP_CMND IUs

[fc] Do not use the command reference number in FCP_CMND IUs

The FCP command reference number is intended to be used for
controlling precise delivery of FCP commands, rather than being an
essentially arbitrary tag field (as with iSCSI and SRP).

Use the Fibre Channel local exchange ID as the tag for FCP commands,
instead of the FCP command reference.  The local exchange ID does not
appear within the FCP IU itself, but does appear within the FC frame
header; debug traces can therefore still be correlated with packet
captures.

Reported-by: Hadar Hen Zion <hadarh@mellanox.co.il>
Signed-off-by: Michael Brown <mcb30@ipxe.org>