net/ena: use unmasked head and tail

net/ena: use unmasked head and tail

The next_to_clean and next_to_use ring pointers sometimes were kept
wrapped around ring size, sometimes used beyond this limit. From now
on we increment them without regard to ring size limits, but when
reading the values they are wrapped accordingly. Moreover unit16_t
are unsed whenever possible.

Signed-off-by: Tal Avraham <talavr@annapurnalabs.com>
Signed-off-by: Jakub Palider <jpa@semihalf.com>