net: stmmac: Correct duplicate if/then/else case found by cppcheck

Cppcheck found a duplicate if/then/else case where a receive descriptor
was being processed. This patch corrects that issue.

cppcheck --force --enable=all --inline-suppr .
...
Checking enh_desc.c...
[enh_desc.c:148] -> [enh_desc.c:144]: (style) Found duplicate if expressions.
...

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>