solos-pci: Fix DMA support

DMA support has finally made its way to the top of the TODO list, having
realised that a Geode using MMIO can't keep up with two ADSL2+ lines
each running at 21Mb/s.

This patch fixes a couple of bugs in the DMA support in the driver, so
once the corresponding FPGA update is complete and tested everything
should work properly.

We weren't storing the currently-transmitting skb, so we were never
unmapping it and never freeing/popping it when the TX was done.
And the addition of pci_set_master() is fairly self-explanatory.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>