net: Google gve: Remove dma_wmb() before ringing doorbell

Current code use dma_wmb() to ensure Rx/Tx descriptors are visible
to device before writing to doorbell.

However, these dma_wmb() are wrong and unnecessary. Therefore,
they should be removed.

iowrite32be() called from gve_rx_write_doorbell()/gve_tx_put_doorbell()
should guaratee that all previous writes to WB/UC memory is visible to
device before the write done by iowrite32be().

E.g. On ARM64, iowrite32be() calls __iowmb() which expands to dma_wmb()
and only then calls __raw_writel().

Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>