mlxsw: pci: Fix the FW ready mask length

The system-status register is actually 16-bit wide and not 8 bit-wide.

Fixes: 233fa44b

 ("mlxsw: pci: Implement reset done check")
Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>