libpayload: Take usb_poll out of usb_initialize.

While it might be slightly more convenient to not have to call usb_poll
manually after calling usb_initialize, you'll still likely want to call it
before trying to use a USB device since one have have been hotplugged since
you last looked. By not calling usb_poll, usb_initialize completes quickly and
can be called unconditionally without a long delay. The delay can be put off
until later when we're sure it's necessary.

Change-Id: Ib8b1bdea996702c42d1b7021f492d9f8e174d304
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/1737
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)

payloads/libpayload/drivers/usb/usbinit.c

@@ -155,6 +155,5 @@ static void usb_scan_pci_bus(int bus)
 int usb_initialize(void)
 {
 	usb_scan_pci_bus(0);
-	usb_poll();
 	return 0;
 }