net: ena: ethtool: convert stat_offset to 64 bit resolution

The type of all stat fields is u64, therefore when iterating over stat
fields in a stats struct, it makes sense to use an offset in 64 bit
resolution. Doing so allows us to drop some of the casting that is
currently used when referencing stats.

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