ixgbe(4): Eliminate bogus sizeof() expressions

All of these uses of sizeof() were on the wrong type in relation to the pointer
passed to SYSCTL_ADD_PROC as arg1. Fortunately, none of the handlers actually
use arg2. So just don't pass a (non-zero) arg2.

Reported by: Coverity
CID: 1007701

(cherry picked from commit d09fbcd0b6eae765a190eaa4dc931050af7bd25f)