staging: mt7621-eth: Fix compilation error

Fix the following comilation error by making sure that "phy_ring_head" is
defined as "dma_addr_t" and change "phy_ring_tail" to match it as both
should be "dma_addr_t".

error: passing argument 3 of ‘dma_alloc_coherent’ from incompatible pointer type

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>