net: ftgmac100: Use module_platform_driver()

module_platform_driver macro removes some boilerplate and makes
the code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Po-Yu Chuang <ratbert@faraday-tech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>