net: ax88796: avoid 64 bit arithmetic

When building ax88796 on an ARM platform with 64-bit resource_size_t,
we currently get

drivers/net/ethernet/8390/ax88796.c:875: undefined reference to `__aeabi_uldivmod'

because we do a division on the length of the MMIO resource.
Since we know that this resource is very short, using an
"unsigned long" instead of "resource_size_t" is entirely
sufficient, and avoids this link-time error.

Cc: Ben Dooks <ben-linux@fluff.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>