backlight: jornada720: Minimise code duplication and handle errors better

Rickard Strandqvi's ran a code analysis application which found that
jornada_lcd_get_contrast(() and jornada_lcd_set_contrast() contained
some code duplication (calling the same functions during various
code execution paths) and failed to return errors in a consistent
manner.  This patch aims to simplify the code, coercing it into
behaving at a level expected of a driver in the Linux kernel.

Suggested-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Lee Jones <lee.jones@linaro.org>