net: bcmgenet: Remove duplicate test for tx_coalesce_usecs_high

We were checking twice for ec->tx_coalesce_usecs_high, remove the
duplicate test.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Reported-by:  <kbuild-all@01.org>
Fixes: 2f913070

 ("net: bcmgenet: Implement TX coalescing control knobs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>