tools, bpf_asm: simplify parser rule for BPF extensions

We can already use yylval in the lexer for encoding the BPF extension
number, so that the parser rules can be further reduced to a single one
for each B/H/W case.

Signed-off-by: Ray Bellis <ray@isc.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>