tracing: Remove size restriction on synthetic event cmd error logging

Currently, synthetic event command error strings are restricted to a
length of MAX_FILTER_STR_VAL (256), which is too short for some
commands already seen in the wild (with cmd strings longer than that
showing up truncated in err_log).

Remove the restriction so that no synthetic event command error string
is ever truncated.

Signed-off-by: Tom Zanussi <zanussi@kernel.org>