x86/PCI: Export find_cap() to be used in early PCI code

maillist inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8X2RA
CVE: NA

Reference: https://lore.kernel.org/linux-pci/20181018183721.27467-2-gpiccoli@canonical.com/



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This patch exports (and renames) the function find_cap() to be used
in the early PCI quirk code, by the next patch.

This is being moved out from AGP code to generic early-PCI code
since it's not AGP-specific and can be used for any PCI device.
No functional changes intended.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>