Print a warning when an unknow USB controller type is detected.

Print a warning when an unknow USB controller type is detected.

The Intel E6XX Atom processor reports an unknown USB controller type (in
addition to the standard EHCI and OHCI ones).  Add a default case to
print a warning when an unknown controller type is detected.

Change-Id: I885d0ccec4c46fd212cceac599290e9bf85edbbb
Signed-off-by: Steven A. Falco <sfalco@coincident.com>
Reviewed-on: http://review.coreboot.org/100
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>