colead-slm: Use sr_dev_inst to store connection handle
fluke-dmm: Use sr_dev_inst to store connection handle
ols: Fix stack clobbering at start of acquisition

This off-by-one was clobbering the stack since introduced in a803c0db4d58.
However it only set one bit, and that generally appears to have been set
already, so this rarely affected anything. But when it did, it affected
a pointer, causing a segfault.
ols: Use sr_dev_inst to store connection handle