[ena] Limit receive queue size to work around hardware bugs

[ena] Limit receive queue size to work around hardware bugs

Commit a801244 ("[ena] Increase receive ring size to 128 entries")
increased the receive ring size to 128 entries (while leaving the fill
level at 16), since using a smaller receive ring caused unexplained
failures on some instance types.

The original hardware bug that resulted in that commit seems to have
been fixed: experiments suggest that the original failure (observed on
a c6i.large instance in eu-west-2) will no longer reproduce when using
a receive ring containing only 16 entries (as was the case prior to
that commit).

Newer generations of the ENA hardware (observed on an m8i.large
instance in eu-south-2) seem to have a new and exciting hardware bug:
these instance types appear to use a hash of the received packet
header to determine which portion of the (out-of-order) receive ring
to use.  If that portion of the ring happens to be empty (e.g. because
only 32 entries of the 128-entry ring are filled at any one time),
then the packet will be silently dropped.

Work around this new hardware bug by reducing the receive ring size
down to the current fill level of 32 entries.  This appears to work on
all current instance types (but has not been exhaustively tested).

Signed-off-by: Michael Brown <mcb30@ipxe.org>