PCI: rockchip: Handle regulator_get_current_limit() failure correctly

regulator_get_current_limit() can return negative error codes.  We saved
the return value in an unsigned "curr", and a subsequent check interpreted
a negative error code as a positive (invalid) current limit.

Save the return code as a signed value, which avoids messages like this,
seen on Samsung Chromebook Plus:

  rockchip-pcie f8000000.pcie: invalid power supply

[bhelgaas: changelog]
Fixes: 4816c4c7 ("PCI: rockchip: Provide captured slot power limit and scale")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Lin <shawn.lin@rock-chips.com>