PCI/bwctrl: Fix pcie_bwctrl_select_speed() return type

PCI/bwctrl: Fix pcie_bwctrl_select_speed() return type

pcie_bwctrl_select_speed() should take __fls() of the speed bit, not return
it as a raw value. Instead of directly returning 2.5GT/s speed bit, simply
assign the fallback speed (2.5GT/s) into supported_speeds variable to share
the normal return path that calls pcie_supported_speeds2target_speed() to
calculate __fls().

This code path is not very likely to execute because
pcie_get_supported_speeds() should provide valid ->supported_speeds but a
spec violating device could fail to synthesize any speed in
pcie_get_supported_speeds(). It could also happen in case the
supported_speeds intersection is empty (also a violation of the current
PCIe specs).

Link: https://lore.kernel.org/r/20250321163103.5145-1-ilpo.jarvinen@linux.intel.com
Fixes: de9a6c8d5dbf ("PCI/bwctrl: Add pcie_set_target_speed() to set PCIe Link Speed")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>