ARM: OMAP2+: use IS_ERR_OR_NULL() helper

ARM: OMAP2+: use IS_ERR_OR_NULL() helper

Simplify error handling in OMAP powerdomain, voltage, and VP code by
replacing open-coded '!ptr || IS_ERR(ptr)' checks with the combined
IS_ERR_OR_NULL() helper.

This improves readability and consistency across
omap_set_pwrdm_state(), voltdm_get_voltage(), voltdm_scale(),
voltdm_reset(), and related functions.

No functional change intended.

Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Reviewed-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20250817083449.2249268-1-yangxiuwei2025@163.com
Signed-off-by: Kevin Hilman <khilman@baylibre.com>