mux: Fix struct mux_state kernel-doc comment

mux: Fix struct mux_state kernel-doc comment

A warning is reported because a colon was dropped, it is found by running
scripts/kernel-doc, which is caused by using 'make W=1'.
drivers/mux/core.c:44: warning: Function parameter or member 'state' not
described in 'mux_state'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/a9a970fc-63f9-5ab9-4983-fba5b01bebe2@axentia.se
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>