i2c: mux: mlxcpld: Prepare mux selection infrastructure for two-byte support

Allow to program register value zero to the mux register, which is
required for word address mux register space support.
Change key selector type from 'unsigned short' to 'integer' in order to
allow to set it to -1 on deselection.
Rename key selector field from 'last_chan' to 'last_val', since this
fields keeps actually selector value and not channel number.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>