PCI: Fix bit definitions of PCI_EXP_LNKCAP2 register

According to the PCIe 3.0 spec, PCI_EXP_LNKCAP2_SLS_2_5GB is
1st bit of PCI_EXP_LNKCAP2 register, not 0th bit. So, the bit
definition of supported link speed vector should be fixed.

[bhelgaas: change "Current" to "Supported"]
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>