patches/openwrt: fix invalid lan when there is none on Mox

Turris Mox can have no lan interface if there is just no switch. The
original code echoed in such case `lan*` instead of nothing.
The new code won't call function to add lan interface unless there is
really some lan interface.
8 jobs for bugfix/invalid-phy in 6 minutes and 38 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Build
passed #523232
lists_generate

00:00:29

passed #523229
medkit_mox

00:06:37

passed #523230
medkit_omnia

00:05:56

passed #523231
medkit_turris1x

00:03:29

passed #523226
prepare_mox

00:02:49

passed #523227
prepare_omnia

00:02:46

passed #523228
prepare_turris1x

00:02:49

 
  Verify
passed #523233
lists_verify

00:00:33