cxgbe: fix module loading

After c03414326909, if_cxgbe.ko fails to load if crypto is not compiled
in kernel, e.g., MINIMAL.

link_elf_obj: symbol hmac_init_ipad undefined
linker_load_file: /boot/kernel/if_cxgbe.ko - unsupported file type
kldload: an error occurred while loading module if_cxgbe. Please check dmesg(8) for more details.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D38482