Cr50: clean up usb_init()
Cr50: clean up usb_init()

No new functionality, just a little refactoring and general
cleanup of the USB initialization steps.

BUG=chrome-os-partner:34893
BRANCH=none
CQ-DEPEND=CL:317376
TEST=make buildall, manual

Connect the Cr50 to my workstation via USB:
* /bin/dmesg reports no errors
* verify EP0 with lsusb -v -d 18d1:5014
* verify EP1 with './extra/usb_console -e 1 -p 5014' (reverses
  case of input text)
* verify EP2 with the 'hid' command on the EC console (types a 'g')

Change-Id: Ia6922acf82a793759870a61217562f4e63608a80
Signed-off-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/317319
Reviewed-by: Dominic Rizzo <a9e5fb9a80871b4edba0a94343af83678d394845@google.com>

akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec