pcap: fix double stop error

pcap: fix double stop error

librte_pmd_pcap driver was opening the pcap/interfaces only at init time and
closing them only when the port was being stopped. This behaviour would cause
problems (leading to segfault) if the user closed the port 2 times. The first
time the pcap/interfaces would be normally closed but libpcap would throw an
error causing a segfault if the closed pcaps/interfaces were closed again.
This behaviour is solved by re-openning pcaps/interfaces when the port is
started (only if these weren't open already for example at init time).

Signed-off-by: Nicolás Pernas Maradei <nico@emutex.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>