iwlwifi: read the Rx write pointer only once

The FH (DMA engine) tells the driver the index of the last
ready (closed) Rx buffer. This data is in closed_rb_num.
If we read this data several times we may get inconsistencies
between the code and the debug prints which can make it
harder to debug issues here.

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