i2c-mux-pca954x: Disable mux after 200ms timeout

Leaving the mux enabled causes needless I2C traffic on the downstream
bus. De-selecting after every request causes excess I2C traffic and
switching.

This patch implements a hybrid solution: After 200ms of inactivity,
the mux is disabled.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>