selftests/livepatch: Replace set_dynamic_debug() with setup_config() in README

Commit 35c9e74c

 ("selftests/livepatch: Make dynamic debug setup and
restore generic") introduced setup_config() to set up the environment
for each test. It superseded set_dynamic_debug().  README still mentions
set_dynamic_debug(), so update it to setup_config() which should be used
now in every test.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>