net: ena: Change the name of bad_csum variable

Changed bad_csum to csum_bad to align with csum_unchecked & csum_good

Signed-off-by: Nati Koler <nkoler@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>