iwlwifi: disable TX AMPDU by default for iwldvm

NICs supported by iwldvm don't handle well TX AMPDU.
Disable it by default, still leave the possibility to
the user to force enable it with a debug parameter.

NICs supported by iwlmvm don't suffer from the same issue,
leave TX AMPDU enabled by default for these.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>