PCI: mvebu: Use FIELD_PREP() with Link Width

mvebu_pcie_setup_hw() setups the Maximum Link Width field in the Link
Capabilities registers using an open-coded variant of FIELD_PREP() with
a literal in shift. Improve readability by using
FIELD_PREP(PCI_EXP_LNKCAP_MLW, ...).

Link: https://lore.kernel.org/r/20230919125648.1920-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>