PCI: brcmstb: Use internal register to change link capability

PCI: brcmstb: Use internal register to change link capability

The driver has been mistakenly writing to a read-only (RO)
configuration space register (PCI_EXP_LNKCAP) to change the
PCIe link capability.

Although harmless in this case, the proper write destination
is an internal register that is reflected by PCI_EXP_LNKCAP.

Thus, fix the brcm_pcie_set_gen() function to correctly update
the link capability.

Fixes: c0452137034b ("PCI: brcmstb: Add Broadcom STB PCIe host controller driver")
Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250214173944.47506-3-james.quinlan@broadcom.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>