mb/google/volteer: Rename remaining pmc_mux/con to conn

mb/google/volteer: Rename remaining pmc_mux/con to conn

CB:43090 renamed con to conn to avoid issues when building on Windows.
CB:42905 introduced more uses of the old name.

Adapt the latter to comply with the former.

Change-Id: I723141add5452fc541f67cb8591793f2d64cc231
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43141
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>