The expression start_idx - dbg_cnt is evaluated using unsigned int
arthithmetic (since these variables are unsigned ints) and hence can never
be less than zero, so the less than comparison is never true.  Rewrite the
expression to check for start_idx being less than dbg_cnt.

After the logic was corrected, temp_idx wasn't working correctly. So fix it
as well.

Link: https://lore.kernel.org/r/20200706204246.130416-1-jsmart2021@gmail.com
Fixes: 372c187b ("scsi: lpfc: Add an internal trace log buffer")
CC: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Addresses-Coverity: ("Unsigned compared against 0")