lspci: Show Slot Power Limit values above EFh

lspci: Show Slot Power Limit values above EFh

PCI Express Base Specification rev. 3.0 has the following definition for
the Slot Power Limit Value:

=======================================================================
When the Slot Power Limit Scale field equals 00b (1.0x) and Slot Power
Limit Value exceeds EFh, the following alternative encodings are used:
  F0h = 250 W Slot Power Limit
  F1h = 275 W Slot Power Limit
  F2h = 300 W Slot Power Limit
  F3h to FFh = Reserved for Slot Power Limit values above 300 W
=======================================================================

Replace function power_limit() by show_power_limit() which also prints
power limit value. Show reserved value as string ">300W".