libpayload: Force checking all EHCI ports on power-on

libpayload: Force checking all EHCI ports on power-on

EHCI port status reporting isn't very consistent on power-on,
so just looking for devices on all ports is the safest way to
find everything.

Change-Id: I26b4305016f0bed1d2c1b5cffc59d5813fa1cbbb
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/594
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>