hw/ufs: Fix code coverity issues

hw/ufs: Fix code coverity issues

Fixed four ufs-related coverity issues.

The coverity issues and fixes are as follows

1. CID 1519042: Security issue with the rand() function
Changed to use a fixed value (0xab) instead of rand() as
the value for testing

2. CID 1519043: Dereference after null check
Removed useless (redundant) null checks

3. CID 1519050: Out-of-bounds access issue
Fix to pass an array type variable to find_first_bit and
find_next_bit using DECLARE_BITMAP()

4. CID 1519051: Out-of-bounds read issue
Fix incorrect range check for lun

Fix coverity CID: 1519042 1519043 1519050 1519051

Signed-off-by: Jeuk Kim <jeuk20.kim@samsung.com>