drivers/amd/pm: smu13: use local variable adev

Since this variable was made available by the previous commit, use
it to make function access cleaner.

Suggested-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>