net: zynq_gem: Add cache flush to zynq_gem_free_pkt

net: zynq_gem: Add cache flush to zynq_gem_free_pkt

Add cache flush to zynq_gem_free_pkt. This is necessary
because some net routines would modify this buffer in place.
The cache_invalidate in the zynq_gem_recv function would cause
the modifications to the buffer to overwrite the DMA from the GEM,
if cache coherency is not enabled in the GEM, the next time the
buffer is in use.

Flushing the cache when the buffer is no longer in use by the
net functions ensures that the GEM DMA is going to take place
into a clean buffer.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>