net/ena: refactor Rx

net/ena: refactor Rx

* Split main Rx function into multiple ones - the body of the main
  was very big and further there were 2 nested loops, which were
  making the code hard to read
* Rework how the Rx mbuf chains are being created - Instead of having
  while loop which has conditional check if it's first segment, handle
  this segment outside the loop and if more fragments are existing,
  process them inside.
* Initialize Rx mbuf using simple function - it's the common thing for
  the 1st and next segments.
* Create structure for Rx buffer to align it with Tx path, other ENA
  drivers and to make the variable name more descriptive - on DPDK, Rx
  buffer must hold only mbuf, so initially array of mbufs was used as
  the buffers. However, it was misleading, as it was named
  "rx_buffer_info". To make it more clear, the structure holding mbuf
  pointer was added and now there is possibility to expand it in the
  future without reworking the driver.
* Remove redundant variables and conditional checks.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>