net/ena: use macros for ring index operations

net/ena: use macros for ring index operations

To improve code readability, abstraction was added for operating on IO
rings indexes.

Driver was defining local variable for ring mask in each function that
needed to operate on the ring indexes. Now it is being stored in the
ring as this value won't change unless size of the ring will change and
macros for advancing indexes using the mask has been added.

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