kingst-la2016: address style issues in api.c scan and open

kingst-la2016: address style issues in api.c scan and open

Improve readability and fixup coding style of the scan(), dev_open(),
and la2016_dev_open() routines. Factor out common subexpressions, trim
text line length, adjust data types, eliminate sizeof() type redundancy.
Propagate returned error codes, check the USB enumeration result before
iteration. Drop the devc member which used to hold the sigrok context.
Release the sample data conversion buffer when acquisition start fails.