xhci: Avoid global symbol pollution with handshake.

Non-static xHCI driver symbols should start with the "xhci_" prefix, in
order to avoid namespace pollution.  Rename the "handshake" function to
"xhci_handshake".

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Ben Hutchings <ben@decadent.org.uk>