The line:
*cmds->cmds[cmd_index].data_len = tx_tmp;
triggered the write to memory, which is declared as read-only (const).
Such writes trigger the memory exception.

To make the code simpler and avoid problematic write,
support only writes that has a size of a register.

The fix was tested for PEC.

Signed-off-by: Adam Wujek <dev_public@wujek.eu>