tests: build_all: adc: bump CONFIG_MAX_THREAD_BYTES

Bump CONFIG_MAX_THREAD_BYTES to 4 to get builds to pass.  Since
we now enable I2C/SPI based if the device needs that bus, we end
up having more thread for all the drivers that get built in.

Since this a build only test there isn't any real issue with doing
this.

Signed-off-by: Kumar Gala <galak@kernel.org>