radioshack-dmm: (Trivial) Convenience fixes

radioshack-dmm: (Trivial) Convenience fixes

While testing the new radioshack-dmm driver with pulseview, I found
a few inconvenients.

1. Print an info message when a port is probed, and when a device is
found. This makes it easy to tell if and where the driver is looking.

2. num_samples was not reset after the first aquisition, so the
second aquisition would quit right away. Reset num_samples at start
of a new aquisition.

3. There's no need to  open the serial port RW, so change O_RDWR to
O_RDONLY when opening the port.

These changes are too trivial to split into different patches.

Signed-off-by: Alexandru Gagniuc <redacted>