net: dsa: mv88e6xxx: don't error out cmode set on missing lane

When the given cmode has no serdes, mv88e6xxx_serdes_get_lane() returns
-NODEV. Earlier in the same function the code skips serdes handing in
this case. Do the same after cmode set.

Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Link: https://lore.kernel.org/r/cd95cf3422ae8daf297a01fa9ec3931b203cdf45.1646050203.git.baruch@tkos.co.il
Signed-off-by: Jakub Kicinski <kuba@kernel.org>