samples: pktgen: correct dev to DEV

In the pktgen_sample01_simple.sh script, the device variable is uppercase
'DEV' instead of lowercase 'dev'. Because of this typo, the script cannot
enable UDP tx checksum.

Fixes: 460a9aa23de6 ("samples: pktgen: add UDP tx checksum support")
Signed-off-by: Wei Fang <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: Jesper Dangaard Brouer <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

samples/pktgen/pktgen_sample01_simple.sh

···
     pg_set $DEV "udp_dst_max $UDP_DST_MAX"
 fi
 
-[ ! -z "$UDP_CSUM" ] && pg_set $dev "flag UDPCSUM"
+[ ! -z "$UDP_CSUM" ] && pg_set $DEV "flag UDPCSUM"
 
 # Setup random UDP port src range
 pg_set $DEV "flag UDPSRC_RND"