atm: horizon: Fix spelling mistakes in TX comment

It's "must not", not "musn't", meaning "shall not".
Let's fix that.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Jun Miao <jun.miao@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>