i2cset: fix 'i' & 's' modes (3 extra bytes were sent)

i2cset: fix 'i' & 's' modes (3 extra bytes were sent)

When 'i' or 's' modes are selected block array is filled from offset 3
(blen = 3) but copied to data.block buffer from offset 0 so first 3 bytes
contains garbage from stack.
The buffer that is sent is also 3 bytes too long due to those extra 3
bytes.

Signed-off-by: Avi Fishman <AviFishman70@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>