pci: use uint32_t for eecp

eecp holds the extended capability offset. If that offset is larger
than 0xff, storing it in uint8_t truncates it, which can make the
early EHCI/XHCI capability walk read the wrong location and loop during
boot.

Seen on AMD device 1022:151e, where HCCPARAMS1 = 0x0118ffc5 and the
first xHCI extended capability offset is 0x460. Widen eecp to uint32_t
in xhci_early_takeover(), matching xhci_pci_take_controller().

Signed-off-by: Gisle Nes <gisle@gisle.net>
Reviewed by: zlei, aokblast
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/2127
Closes: https://github.com/freebsd/freebsd-src/pull/2127