net: dsa: lan9303: Correct register names in comments

Two comments refer to registers, but lack the LAN9303_ prefix.
Fix that.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>