net: fec: fix build warnings for 64bits support

When building for 64bits system, we get some warnings about type
cast between pointer and integer. This patch eliminates the warnings
by using ulong/long type which is 32bits on 32bits system or 64bits on
64bits system.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>