app/pdump: fix type casting of ring size

ring_size value is wrongly type casted to uint16_t.
It should be type casted to uint32_t, as maximum
ring size is 28bit long. Wrong type cast
wrapping around the ring size values bigger than 65535.

Fixes: caa7028276b8 ("app/pdump: add tool for packet capturing")

Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>