scripts: merge_config: Fix typo in variable name.

${WARNOVERRIDE} was misspelled as ${WARNOVVERIDE}, which caused a shell
syntax error in certain paths of the script execution.

Fixes: 46dff8d7

 ("scripts: merge_config: Add option to suppress warning on overrides")
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>