Altera TSE: Correct typecast issue detected by kbuild test robot

This patch addresses a portable pointer arithmetic issue in the
original submission found by the kbuild test robot.

config: make ARCH=i386 allyesconfig

   altera_sgdma.c: In function 'sgdma_txphysaddr':
>> altera_sgdma.c:393:33: warning: cast from
>> pointer to integer of different size [-Wpointer-to-int-cast]
     dma_addr_t offs = (dma_addr_t)((dma_addr_t)desc -
                                    ^
>> altera_sgdma.c:394:5: warning: cast from
>> pointer to integer of different size [-Wpointer-to-int-cast]
        (dma_addr_t)priv->tx_dma_desc);
        ^
   altera_sgdma.c: In function 'sgdma_rxphysaddr':
>> altera_sgdma.c:403:33: warning: cast from
>> pointer to integer of different size [-Wpointer-to-int-cast]
     dma_addr_t offs = (dma_addr_t)((dma_addr_t)desc -
                                    ^
>> altera_sgdma.c:404:5: warning: cast from
>> pointer to integer of different size [-Wpointer-to-int-cast]
        (dma_addr_t)priv->rx_dma_desc);
        ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>