w1: mxc_w1: Add warning for base frequency calculation

Base frequency should be as close as possible to 1 MHz. This patch
adds warnings when clock is unreliable, according to i.MX datasheet.

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>