mlxsw: reg: Remove unused function argument

The 'lock' argument is always set to the default value of '0'. Remove it
from the arguments list.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/fb5dd22830622ceeda1c2d6431c27fccd0687aca.1690281940.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>