mtip32xx: fix potential crash on SEC_ERASE_UNIT

The mtip driver lifted this code from elsewhere and then added a special
handling check for SEC_ERASE_UNIT. If the caller tries to do a security
erase but passes no output data for the command then outbuf is not
allocated and the driver duly explodes.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Selvan Mani <smani@micron.com>
Signed-off-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>