Replace dead OMAP_MUX_ERRORS with OMAP_MUX_WARNINGS

OMAP_MUX_ERRORS doesn't exist in Kconfig, therefore replacing all
references for it with OMAP_MUX_WARNINGS in the source code.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Tony Lindgren <tony@atomide.com>