bgpd: fix SNMP write support

bgpd: fix SNMP write support

This code - dating back to the initial import in 2002 - probably never
worked.  Calling asn_parse_int seems to always have been wrong, and in
the meantime, there no longer is a "struct variable *" argument for
write_method.  If anyone tried to use it, it'd probably have crashed.
(I didn't try.)

Fix this up so it actually works.  It's the only place in Quagga where a
SNMP write is actually supported, so it's an odd one out anyway, but
heh.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit d689d1a0c69726330d69b2dd412fdb8dcb23394b)