scsi: ufs: add queue fullness statistics

Add more statistics to allow tracking of tags occupancy
upon sending a new request. The statistics is kept separately
for 4 types of requests: read, write, urgent and flush.
All will consist only of data requests (eg. read, write, urgent).
This statistic is an enhancement of current tag statistic and
uses same infrastructure.

Change-Id: If5cea4aec4e94081d568a3661584b31665becfc6
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>