[SCSI] qla2xxx: Return blank sysfs string on initial get thermal failure.

When thermal temperature initially fails, return a blank string to the
sysfs interface.  This fixes the initial display of 0.00 followed by
subsequent display of blank line; the initial 0.00 should have not
displayed for cards that do not support thermal temperature.

Signed-off-by: Joe Carnuccio <joe.carnuccio@qlogic.com>
Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>