scsi: megaraid: Fix set but unused variable

In megadev_ioctl(), if MEGA_HAVE_STATS is not defined, the variables
num_ldrv and ustats are unused. Conditionally define them to avoid compiler
warnings.

Link: https://lore.kernel.org/r/20200706123351.451959-1-damien.lemoal@wdc.com


Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>