gdrom: fix compilation error

Use the right name for the struct request variable that removes the
following compilation error :-

make --silent --keep-going --jobs=8
O=/home/tuxbuild/.cache/tuxmake/builds/1/tmp ARCH=sh
CROSS_COMPILE=sh4-linux-gnu- 'CC=sccache sh4-linux-gnu-gcc'
'HOSTCC=sccache gcc'

In file included from /builds/linux/include/linux/scatterlist.h:9,
                 from /builds/linux/include/linux/dma-mapping.h:10,
                 from /builds/linux/drivers/cdrom/gdrom.c:16:
/builds/linux/drivers/cdrom/gdrom.c: In function 'gdrom_readdisk_dma':
/builds/linux/drivers/cdrom/gdrom.c:586:61: error: 'rq' undeclared
(first use in this function)
  586 |  __raw_writel(page_to_phys(bio_page(req->bio)) + bio_offset(rq->bio),
      |                                                             ^~

Fixes: 1d2c8200

 ("gdrom: support highmem")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>