IB/mthca: Factor common MAD initialization code

Factor out common code for initializing MAD packets, which is shared
by many query routines in mthca_provider.c, into init_query_mad().

add/remove: 1/0 grow/shrink: 0/4 up/down: 16/-44 (-28)
function                                     old     new   delta
init_query_mad                                 -      16     +16
mthca_query_port                             521     518      -3
mthca_query_pkey                             301     294      -7
mthca_query_device                           648     641      -7
mthca_query_gid                              453     426     -27

Signed-off-by: Roland Dreier <rolandd@cisco.com>