ARM: pxa: hx4700: Remove unused field initializers

The ID and period for the backlight PWM are obtained from a PWM lookup
table, so the corresponding values don't need to be duplicated into the
platform data.

Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>