openbench-logic-sniffer/protocol: Fix hardcoded unitsize

openbench-logic-sniffer/protocol: Fix hardcoded unitsize

Some output modules, like srzip, calculate unitsize from maximum number of
channels in the session; and throw an error if the actual datastream uses
bigger size. Remove hardcoded value of 4 and calculate it from maximum number
of channels, supported by the device. Other driver seem to do the same.

This fixes "Unexpected unit size" saving a capture, made by an OLS device with
small number of channels.

The bug was found by using gillham's Arduino analyzer
(https://github.com/gillham/logic_analyzer), which supports a maximum of 8
channels. According to the error message, srzip expected unitsize == 1.

Signed-off-by: Pavel Fedin <redacted>