selftests: firmware: Fix ignored return val of asprintf() warn

selftests: firmware: Fix ignored return val of asprintf() warn

Fix the following ingonred return val of asprintf() warn during
build:

cc -Wall -O2    fw_namespace.c  -o ../tools/testing/selftests/firmware/fw_namespace
fw_namespace.c: In function ‘main’:
fw_namespace.c:132:2: warning: ignoring return value of ‘asprintf’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  132 |  asprintf(&fw_path, "/lib/firmware/%s", fw_name);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210708031827.51293-1-skhan@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>