net: ftmac100: Cast priv->iobase with uintptr_t

priv->iobase was declared as phys_addr_t which is now a 64-bit
address. In a 32-bit build, this causes the following warning
seen when building ftmac100.c:

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

Cast priv->iobase with uintptr_t.

Signed-off-by: Bin Meng <bin.meng@windriver.com>