mlxsw: core_env: Split module power mode setting to a separate function

Move the code that applies the module power mode to the device to a
separate function. This function will be invoked by the next patch to
set the power mode on transceiver modules found on a line card when the
line card becomes active.

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