vmxnet3: Add locking for access to command register

Access to cmd register is racey, especially in smp environments. Protect
it using a spinlock.

Signed-off-by: Matthieu Bucchianeri <matthieu@vmware.com>
Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>