vmxnet3: round down # of queues to power of two

vmxnet3 device supports only power-of-two number of queues. The driver
therefore needs to check this and rounds down the number of queues to the
nearest power of two.

Signed-off-by: Yong Wang <yongwang@vmware.com>
Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Reviewed-by: Dmitry Torokhov <dtor@vmware.com>