Fix decoding of "UTF-8" registers

[lowar]

This patch fixes the decoding of UTF-8 values ("serial_number", "dsp_software_version", "arm_software_version")

Since the expected length of the value is not known at decoding time, the maximum length of the string needs to be provided in the "register-sungrow.yaml" as a "length" key with the "UTF-8" datatype:

     - name: "serial_number"
       level: 3
       address: 4990
       datatype: "UTF-8"
       length: 10

Downward compatibility is ensured by using a default length of 5.

[lowar]

This would fix issue #8