kingst-la2016: address endianess issue in data feed submission

kingst-la2016: address endianess issue in data feed submission

Capture data was taken from the device in an endianess aware manner, but
submission to the session feed assumed a little endian host (and casted
an u8 pointer to u16 items, which is bad). Use an endianess aware writer
before sending accumulated sample data to the sigrok session. Address
remaining open coded byte counts in several send_chunk() places.

The conversion from LE (the device) to the host and to LE (the session)
may appear redundant, but is worth keeping in place when other models
with 32 channels should get added in a future implementation.