radioshack-dmm: Fix opening of serial port.

During scan the serial port is opened with SERIAL_RDONLY | SERIAL_NONBLOCK,
which works fine, but when acquisition starts, it is opened only with
SERIAL_RDONLY. On Linux, if cdc_acm can make a claim to the USB to serial
converter, opening the port will fail.

Open port with SERIAL_RDONLY | SERIAL_NONBLOCK.

Signed-off-by: Alexandru Gagniuc <redacted>