usbmon vs. tcpdump: fix dropped packet count

Report the number of dropped packets instead of zero
when using the binary usbmon interface with tcpdump.

# tcpdump -i usbmon1 -w dump
tcpdump: listening on usbmon1, link-type USB_LINUX_MMAPPED (USB with padded Linux header), capture size 65535 bytes
^C2155 packets captured
2155 packets received by filter
1019 packets dropped by kernel

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>