ide: Make ide_cdrom_prep_fs() initialize the sense buffer pointer

The changes introduced through commit 82ed4db4 assume that the
sense buffer pointer in struct scsi_request is initialized for all
requests - passthrough and filesystem requests. Hence make sure
that that pointer is initialized for filesystem requests. Remove
the memset() call that clears .cmd because the scsi_req_init()
call in ide_initialize_rq() already initializes the .cmd.

Fixes: commit 82ed4db4 ("block: split scsi_request out of struct request")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>