Input: snvs_pwrkey - use local device pointer to simple code

Use local struct device pointer to avoid reference the platform_device
pointer every time.

No functional change.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Joy Zou <joy.zou@nxp.com>
Link: https://patch.msgid.link/20260715-b4-pwrkey-v5-3-07e7353c319e@oss.nxp.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>