Revert "tasks/task-mcuio.c: fix trivial warning (unused variable nranges)"

This reverts commit 5932c6baec7942bcd9b790f22115fba8010243d6.
mcuio_bitbang_i2c_func: fix smbus read block command

When input length is -1, a read block cmd is being issued, take the first
input byte as input buffer length.