Removing dead code in drivers/net/wan/hdlc_fr.c

The local variable "prefix" is never used anymore, and the content of
this string appears a bit later, directly in a call to "alloc_netdev"
after doing exactly the same if/else test. So there seems to be no
point keeping those 4 lines anymore.

Signed-off-by: Cédric Augonnet <cedric.augonnet@ens-lyon.org>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>