PCI: Bring the PCIe speed to MBps logic to new pcie_dev_speed_mbps()

Bring the switch case in pcie_link_speed_mbps() to new function to
the header file so that it can be used in other places like
in controller driver.

Link: https://lore.kernel.org/linux-pci/20240619-opp_support-v15-3-aa769a2173a3@quicinc.com


Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>