net: ftmac100: simplify priv->iobase casting

Replace 'phys_addr_t iobase' with 'struct ftmac100 *ftmac100' in
struct ftmac100_data. It allows to remove casting in a number of places.

Since priv->iobase is phys_addr_t, use phys_to_virt() to make
a pointer from it.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>