net: ena: Use dev_alloc() in RX buffer allocation

Use dev_alloc() when allocating RX buffers instead of specifying the
allocation flags explicitly. This result in same behaviour with less
code.

Also move the page allocation and its DMA mapping into a function. This
creates a logical block, which may help understanding the code.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>