ftrace: Remove an unneeded NULL check

"func" can't be NULL and it doesn't make sense to check because we've
already derefenced it.

Link: http://lkml.kernel.org/r/20170712073340.4enzeojeoupuds5a@mwanda



Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>