iscsi_ibft: search for broadcom specific ibft sign (v2)

Broadcom iscsi offload firmware uses a non standard ibft sign of "BIFT".
When we added support for boot, the anaconda team and I were using
older firmware (I guess 4 years old), so boot does not work on current
cards.

This patch modifies the ibft search code to search for "BIFT" along
with the other possible values.

Broadcom has tested the patch and reported it works with their
firmware. Mike has tested Chelsio and Intel cards.

[v2:
- Add ACPI_SIG_IBFT to ibft_signs
- replace break with goto in find_ibft_in_mem innner loop.]
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>