Let the xhci_hw_root structure span exactly XHCI_PAGE_SIZE bytes by increasing
the number of completion event TRBs. This avoids wasting memory.

Sponsored by: NVIDIA Networking
Approved by: re (gjb)

(cherry picked from commit e036ee6ce2e4d6fe0564f8bb3aa99a2f61d48fc6)
(cherry picked from commit afeeccfbed271f4f6ba7b300d01eea34281395fd)