net: wan: sdla: Fix cast from pointer to integer of different size

Since net_device.mem_start is unsigned long, it should not be cast to
int right before casting to pointer.  This fixes warning (compile
testing on alpha architecture):

    drivers/net/wan/sdla.c: In function ‘sdla_transmit’:
    drivers/net/wan/sdla.c:711:13: warning:
        cast to pointer from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>