iwlwifi: pcie: unify iwl_rx_replenish and iwl_rx_replenish_now

Besides the different allocation flags, they are really
the same. Pass the gfp_t flags as a parameter, and unify
them.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>