gianfar: Fix potential oops during OF address translation

gianfar driver may pass NULL pointer to the of_translate_address(),
which may lead to a kernel oops. Fix this by using of_iomap(), which
is also much simpler and shorter.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>