i2c: mux: demux-pinctrl: check initial mux selection, too

When misconfigured, the initial setup of the current mux channel can
fail, too. It must be checked as well.

Fixes: 50a5ba87 ("i2c: mux: demux-pinctrl: add driver")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>