phylib: simplify NULL checks

Fix scripts/checkpatch.pl's messages like:

CHECK: Comparison to NULL could be written "!phydrv->read_mmd_indirect"

BTW, it doesn't detect the reversed comparisons (which I've fixed as well).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>