scsi: ufs: fix's to ufs_lun_depth unit test

This patch fixes the ufs unit test case "lun_depth" to work properly.
In cases where nutrs == lun_qdepth, 0 was an argument for the random
number generating function, which returned bogus numbers as a result.
In this patch we distinguish between equality and inequality cases, where
in the equality case we remove the irrelevant test cases.

Change-Id: Id7967e894e9d4d0fa1e250852895debcefcbf818
Signed-off-by: Lee Susman <lsusman@codeaurora.org>