Revert "xhci: replace xhci_read_64() with readq()"

This reverts commit e8b37332.  Many xHCI
host controllers can only handle 32-bit addresses, and writing 64-bits
at a time causes them to fail.  Reading 64-bits at a time may also cause
them to return 0xffffffff, so revert this commit as well.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>