USB backend: Append USB iSerial to context description
USB backend: Append USB iSerial to context description

Signed-off-by: Michael Hennerich <38f18d03059a1bc88082b55d7ca2d9b1ebfaffe0@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio