net/enetc: do not stall in clean Tx ring

Don't read the hardware CI register in a loop, read it once, clean up
and exit.
The issue with reading the register in a loop is that we're stalling
here trying to catch up with hardware which keeps sending traffic as
long as it has traffic to send, so in effect we could be waiting here
for the Tx ring to be drained by hardware, instead of us doing Rx in
that meantime.
At the time we return the function there may be new BDs in the ring that
could be cleaned, we're just leaving those there for the next time.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Gagandeep Singh <g.singh@nxp.com>