mlxsw: core: Reduce buffer size in transactions for SFP modules temperature readout

Obtain SFP modules temperatures through MTMP register instead of MTBR
register, because the first one utilizes shorter transaction buffer size
for request. It improves performance in case low frequency interface
(I2C) is used for communication with a chip.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>