mtip32xx: fix user_buffer check in exec_drive_command

Current user_buffer check is incorrect and causes hdparm to fail

# hdparm -I /dev/rssda
 HDIO_DRIVE_CMD(identify) failed: Input/output error

/dev/rssda:

Patching linux-3.6-rc5 hdparm works as expected

# hdparm -I /dev/rssda
/dev/rssda:

ATA device, with non-removable media
	Model Number:       DELL_P320h-MTFDGAL350SAH
	Serial Number:      00000000121302025F01
	Firmware Revision:  B1442808
<snip>

Reported-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: David Milburn <dmilburn@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>