PCI: mediatek-gen3: Split out device power helpers

PCI: mediatek-gen3: Split out device power helpers

In preparation for adding full power on/off control with the pwrctrl API,
split out the existing code that only partially deals with device power
sequencing into separate helper functions. The existing code only handles
PERST#.

This is purely moving code around, and brings no functional changes.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
[mani: moved the 'err' variable to next commit]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20260324052002.4072430-6-wenst@chromium.org