usb_updater2: Read bInterfaceNumber correctly.
usb_updater2: Read bInterfaceNumber correctly.

This patch makes find_interface use bInterfaceNumber explicitly instead
of the i iterator, which could be different from the discovered
interface number if there is no interface at index 0.

BUG=b:229012657
BRANCH=None
TEST=Run 'usb_updater2 -f' on Vell.

Change-Id: I7e156eefad6729b5d568fb0e0308b93cdb78a2ea
Signed-off-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3584693
Reviewed-by: Parth Malkan <c3d09dcbffc3a0a63125afed46f55e5cbf5bb3a9@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec