sr_usb_find(): Increase the 'bus' limit to 255.

sr_usb_find(): Increase the 'bus' limit to 255.

On some systems it can happen that the USB 'bus' number is a lot larger
than 64, but sr_usb_find() currently errors out if it is > 64.

Example:
Bus 250 Device 006: ID 1ab1:04ce 1ab1 DS1000Z Series[...]

Increase that limit so that the code will work everywhere. This bus number
is queried via libusb_get_bus_number() which returns an uint8_t, so we're
limiting to 255 here.

Thanks to 'ssi' on IRC for reporting the issue.