[util/genbnx2] Search for proper MIPS FW header location

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>
[driver/net/bnx2_fw.h] Fixup firmware for BCM5706 due to wrong COM and RXP header location

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>