pktgen: Fix inconsistent of format with argument type in pktgen.c

Fix follow warnings:
[net/core/pktgen.c:925]: (warning) %u in format string (no. 1)
	requires 'unsigned int' but the argument type is 'signed int'.
[net/core/pktgen.c:942]: (warning) %u in format string (no. 1)
	requires 'unsigned int' but the argument type is 'signed int'.
[net/core/pktgen.c:962]: (warning) %u in format string (no. 1)
	requires 'unsigned int' but the argument type is 'signed int'.
[net/core/pktgen.c:984]: (warning) %u in format string (no. 1)
	requires 'unsigned int' but the argument type is 'signed int'.
[net/core/pktgen.c:1149]: (warning) %d in format string (no. 1)
	requires 'int' but the argument type is 'unsigned int'.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>