mkiss: remove redundant assignment of len to ax->mtu

Variable len is being assigned a value that is never read,
hence the assignment is redundant and can be removed. Cleans
up clang warning:

drivers/net/hamradio/mkiss.c:443:3: warning: Value stored to
'len' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>