iwlwifi: continue clean up - pcie/tx.c

Rename static functions. Function moved from trans.c to
tx.c. A few could be made static, others had to be exported.

Functions that implement the transport API are prefixed by
iwl_trans_pcie_, the others by iwl_pcie_.

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