pwm: sun4i: Narrow scope of local variable

The variable pval is only used in a single block in the function
sun4i_pwm_calculate(). So declare it in a more local scope to simplify
the function for humans and compilers.

While at it also simplify assignment to pval.

While the diffstat for this patch is negative for this patch I still
thing the advantage of having a narrower scope is beneficial.

In my compiler / .config setup (gcc 8.2.1, arm/imx_v6_v7_defconfig +
COMPILE_TEST + PWM_SUN4I) this change doesn't result in any binary
changes.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>