iwlwifi: store default station flags in context

Since the default context is initialised to zero,
and the default flags are zero, no more code is
needed to initialise them right now, but another
context can have different default flags.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>