mlxsw: reg: Limit MTBR register payload to a single data record

The MTBR register is used to read temperatures from multiple sensors in
one transaction, but the driver only reads from a single sensor in each
transaction.

Rrestrict the payload size of the MTBR register to prevent the
transmission of redundant data to the firmware.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>