net: ftmac100: Fix compiler warning

Fix warnings as below when compile in 64-bit.

warning: cast from pointer to integer
of different size [-Wpointer-to-int-cast]

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Cc: Greentime Hu <green.hu@gmail.com>