ixgbe: Clarify index name in ixgbe_mc_filter_apply

"It looks like it would be less confusing to rename 'count' to
something like 'idx', since that's what it's used for in this
function."

Reviewed by: erj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29798

(cherry picked from commit 21afed4b1d18578aa8c9fa31e9e677971f8b4300)