netdev-dpdk: Configure flow control only when necessary.

netdev-dpdk: Configure flow control only when necessary.

It is not necessary to touch the physical device each time, if the
configuration has not been changed. Also, few style issues fixed.

Thread-safety annotation added to 'dpdk_set_rxq_config()'. It was
missed while previous refactoring of the flow control configuration.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Tested-by: Sugesh Chandran <sugesh.chandran@intel.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>