net: bcmgenet: Do not return from void function

A stray return was added in the macro bcmgenet_##name##_writel where it
should not, drop it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 69d2ea9c ("net: bcmgenet: Use correct I/O accessors")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>