mkiss: remove redundant check for len > 0

The check for len > 0 is always true and hence is redundant as
this check is already being made to execute the code inside the
while-loop. Hence it is redundant and can be removed.

Cleans up cppcheck warning:
drivers/net/hamradio/mkiss.c:220: (warning) Identical inner 'if'
condition is always true.

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