Fix TypeError in the multi-register write_sensor_register path

[nikicat]

length is len(address), an int, so iterating it raises:

length = len(address)
...
for i in length:          # TypeError: 'int' object is not iterable

The single-register path (address passed as an int) is the only one any driver in this repo uses, which is why this has gone unnoticed. Passing a list — the documented alternative, per the int | list[int] annotation — fails immediately:

>>> device.write_sensor_register([0x0220, 0x0236], [b"\x78\x0b", b"\xb9\x00"])
TypeError: 'int' object is not iterable

With range(length) it builds the expected payload:

01 20 02 78 0b 36 02 b9 00

Not exercised against hardware — I have no driver that writes multiple registers in one call — but the previous code could not run at all, so this can only be an improvement.

Independent of #72 and #73; branches from master.