i2c: muxes: i2c-arb-gpio-challenge: Demote non-conformant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/i2c/muxes/i2c-arb-gpio-challenge.c:43: warning: Function parameter or member 'muxc' not described in 'i2c_arbitrator_select'
 drivers/i2c/muxes/i2c-arb-gpio-challenge.c:43: warning: Function parameter or member 'chan' not described in 'i2c_arbitrator_select'
 drivers/i2c/muxes/i2c-arb-gpio-challenge.c:86: warning: Function parameter or member 'muxc' not described in 'i2c_arbitrator_deselect'
 drivers/i2c/muxes/i2c-arb-gpio-challenge.c:86: warning: Function parameter or member 'chan' not described in 'i2c_arbitrator_deselect'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>