Exynos: Tidy up the pwm_config function in the exynos pwm driver

Exynos: Tidy up the pwm_config function in the exynos pwm driver

Some small fixes in the exynos pwm driver:

1. NS_IN_HZ is non-sensical since these are not compatible units. This
constant actually describes the number of nanoseconds in a second. Renamed it
to NS_IN_SEC. Also dropped the unnecessary parenthesis.
2. The variable "period" is not used to hold a period, it's used to hold a
frequency. Renamed it to "frequency".
3. tcmp is an unsigned value, so (tcmp < 0) will never be true and the if
which checks that condition will never execute. Also, there should be no
problem if the pwm never switches, so there's no reason to subtract one from
tcmp and therefore no reason to compare it against zero. Removed both ifs. If
they weren't removed, tcmp should be a signed value.
4. Add a check for a 0 period.

Test with command "sf probe 1:0; time sf read 40008000 0 1000".
Try with different numbers of bytes and see that sane values are obtained
Build and boot U-boot with this patch, backlight works properly.

Signed-off-by: Gabe Black <gabeblack@google.com>
Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>