mlxsw: reg: Fix SPVMLR max record count

The num_rec field is 8 bit, so the maximal count number is 255.
This fixes vlans learning not being enabled for wider ranges than 255.

Fixes: a4feea74

 ("mlxsw: reg: Add Switch Port VLAN MAC Learning register definition")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>