net/ena/base: simplify loop copying Rx descriptors

net/ena/base: simplify loop copying Rx descriptors

Checking for the cdesc not being NULL doesn't have any sense if the idx
argument is not 0, so it can be skipped, as the error won't be detected
anyway.

To simplify that, only the 'i' value is being verified and the code is
breaking from the infinite loop in case when all descriptors were copied
into the buffer.

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