m_mirred: don't bail if the control action is missing

m_mirred: don't bail if the control action is missing

The mirred act admits an optional control action, defaulting
to TC_ACT_PIPE. The parsing code currently emits an error message
if the control action is not provided on the command line, even
if the command itself completes with no error.

This change shuts down the error message, using the appropriate
parsing helper.

Fixes: e67aba559581 ("tc: actions: add helpers to parse and print control actions")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>