NET: cdc-phonet, handle empty phonet header

Currently, for N 5800 XM I get:
cdc_phonet: probe of 1-6:1.10 failed with error -22

It's because phonet_header is empty. Extra altsetting looks like
there:
E 05 24 00 01 10 03 24 ab 05 24 06 0a 0b 04 24 fd  .$....$..$....$.
E 00                                               .

I don't see the header used anywhere so just check if the phonet
descriptor is there, not the structure itself.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>