net/vmxnet3: fix debug macro pattern for vmxnet3

Vmxnet3 uses the following debug macro style:

 #ifdef SOME_DEBUG
 #  define debug(...) do{ printf(...); } while (0)
 # else
 #  define debug(...) do{ } while (0)
 #endif

If SOME_DEBUG is undefined, then format string inside the
debug macro will never be checked by compiler. Code is
likely to break in the future when SOME_DEBUG is enabled
 because of lack of testing. This patch changes this
to the following:

 #define debug(...) \
  do { if (SOME_DEBUG_ENABLED) printf(...); } while (0)

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
