ixgbe: Fix typo in ixgbe_common.h

Correct spelling "packtetbuf_num" to "packetbuf_num" in
drivers/net/ethernet/intel/ixgbe/ixgbe_common.h

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>