iwlwifi: pcie: clean up ICT allocation code

Since iwl_trans_pcie_alloc_ict is called in the PCIe
allocation code, we always set CSR_INT_BIT_RX_PERIODIC.
Move that bit to the default list of interrupts we enable
and simplify the code.
Also use dma_zalloc_ and avoid to memset the memory
afterwards.
trans_pcie->ict_index is 0 since trans_pcie has just been
kzalloced, remove the redundant assignment.

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