w1: mxc_w1: Enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.
To do this, __raw_{read,write}b() functions was be replaced with
simple {read,write}b() variants.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>