kconfig/symbol.c: handle choice_values that depend on 'm' symbols

If choices consist of choice_values of type tristate that depend on
symbols set to 'm', those choice_values are not set to 'n' if the
choice is changed from 'm' to 'y' (in which case only one active
choice_value is allowed). Those values are also written to the config
file causing modules to be built when they should not.

The following config can be used to reproduce and examine the problem;
with the frontend of your choice set "Choice 0" and "Choice 1" to 'm',
then set "Tristate Choice" to 'y' and save the configuration:

config modules
	boolean modules
	default y
	option modules

config dependency
	tristate "Dependency"
	default m

choice
	prompt "Tristate Choice"
	default choice0

config choice0
	tristate "Choice 0"

config choice1
	tristate "Choice 1"
	depends on dependency

endchoice

This patch sets tristate choice_values' visibility that depend on
symbols set to 'm' to 'n' if the corresponding choice is set to 'y'.

This makes them disappear from the choice list and will also cause the
choice_values' value set to 'n' in sym_calc_value() and as a result
they are written as "not set" to the resulting .config file.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Gouders <dirk@gouders.net>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Michal Marek <mmarek@suse.com>