package/busybox: enable ttyUSB[0-9] when using mdev

Some of our systems are using ttyUSB0 devices as a serial console.
Add them to mdev.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>