scsi: ufs: add read string descriptor api

The string descriptor may consist of up to 126 UNICODE characters.
The number of UNICODE characters is calculated
by (descriptor's length - 2)/2.
In addition the api allows to get the result in an ascii format.

Change-Id: If43453bf5f9a826900c0acc064e8def275c0f7d1
Signed-off-by: Raviv Shvili <rshvili@codeaurora.org>