net: ena: ethtool: add extra properties retrieval via get_priv_flags

This commit adds a mechanism for exposing different device
properties via ethtool's priv_flags. The strings are provided
by the device and copied to user space through the driver.

In this commit we:

Add commands, structs and defines necessary for handling
extra properties

Add functions for:
Allocation/destruction of a buffer for extra properties strings.
Retreival of extra properties strings and flags from the network device.

Handle the allocation of a buffer for extra properties strings.

* Initialize buffer with extra properties strings from the
  network device at driver startup.

Use ethtool's get_priv_flags to expose extra properties of
the ENA device

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>