iwlwifi: mvm: fix harmless smatch / coccinelle warnings

Don't check if mvm->fw->cs is NULL since it can't be.
cs is an array member of iwl_fw, it can't be NULL.

Use memset(ptr, 0, sizeof(*ptr)); instead of
memset(ptr, 0, sizeof(struct ptr_type));

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