iscsi_ibft: Don't limits Targets and NICs to two

According to iSCSI Boot Firmware Table Version 1.03 [1], the length of
the control table is ">= 18", where the optional expansion structure
pointer follow the mandatory ones. This allows for more than two NICs
and Targets.

[1] ftp://ftp.software.ibm.com/systems/support/bladecenter/iscsi_boot_firmware_table_v1.03.pdf



Let's enforce the minimum length of the control structure instead
instead of limiting it to the smallest allowed size.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@darnok.org>