cxgb4: make function ch_flower_stats_cb, fixes warning

The function ch_flower_stats_cb is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warnings:
symbol 'ch_flower_stats_cb' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>