tools: bpftool: fold hex keyword in command help

Instead of spelling [hex] BYTES everywhere use DATA as keyword
for generalized value.  This will help us keep the messages
concise when longer command are added in the future.  It will
also be useful once BTF support comes.  We will only have to
change the definition of DATA.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>