PCI/sysfs: Add __weak pci_legacy_has_sparse() helper

PCI/sysfs: Add __weak pci_legacy_has_sparse() helper

Currently, Alpha's sparse/dense legacy attribute handling is done via
pci_adjust_legacy_attr(), which updates dynamically allocated attributes at
runtime.  The upcoming conversion to static attributes needs a way to
determine sparse support at visibility check time.

Add a __weak pci_legacy_has_sparse() that returns false by default.  Alpha
overrides it to check has_sparse() on the bus host controller.

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Link: https://patch.msgid.link/20260508043543.217179-22-kwilczynski@kernel.org