kbuild: remove remaining use of undefined YACC_PREFIX

Commit eea199b4 ("kbuild: remove unnecessary LEX_PREFIX and
YACC_PREFIX") removed YACC_PREFIX definition, but left one use of it. There
was not any build error since there is no user of "cmd_bison_h" currently.
Remove the last remaining occurrence of YACC_PREFIX.

Fixes: eea199b4 ("kbuild: remove unnecessary LEX_PREFIX and YACC_PREFIX")
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>