w1: mxc_w1: Remove unused field "clkdiv" from private structure

Private field "clkdiv" is not used outside "probe", so there are
no reason to keep it in driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>