iwlwifi: move prph handling into the transport

New transports may handle it internally for better performance.
Also move the tracing inside PRPH access which will make the
output more readable:

iwlwifi_dev_ioread_prph32: Read 0x0 from SCD_AGGR_SEL (32-bit)

instead of the corresponding accesses to HBUS_TARG_PRPH_*.

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