i2c: allow shell commands to operate on up to 32-bit registers

Improve shell commands to support up to 32-bit addresses.
The check if function should use 8/16/24/32-bit address is basing
on the length of address used in function parameter. To address
registers below 256 using 16-bit, one should write them left-padded
with zeroes. Address is sent in big endian.

Signed-off-by: Michał Barnaś <mb@semihalf.com>