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: 460a9aa2 ("samples: pktgen: add UDP tx checksum support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/20241112030347.1849335-1-wei.fang@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>