Rename usb.c to ehci.c since it contains EHCI specific content.
TEST=Build and run on Galileo Gen2
Change-Id: Ifdb7cd937b1dffda1959b76e1c911ffd93f53cb6
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/14939
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@google.com>