iwlwifi: continue clean up - pcie/rx.c

Rename static functions. Function moved from trans.c to
rx.c. A few could be made static, others had to be exported.
Also, don't use rxb or rxbuf, but rb which stands for receive
buffer.

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