usb: Expand buffer size in usb_find_and_bind_driver()

The "generic_bus_%x_dev_%x" string which is printed into this buffer
can be up to 34 characters long ("generic_bus_12345678_dev_12345678").
The buffer would be clipped by snprintf() if both %x were at maximum
range. Make sure the buffer is long enough to cover such possibility.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>