ixl(4): Remove compile warning for unused function.

sys/dev/ixl/if_ixl.c:4377:1: warning: unused function 'ixl_debug_info' [-Wunused-function]
ixl_debug_info(SYSCTL_HANDLER_ARGS)

Differential Revision: https://reviews.freebsd.org/D3718
Submitted by: bz