ide-cd: fix DMA alignment regression

e5318b53 ("ide: use the dma safe check for
REQ_TYPE_ATA_PC") introduced a regression which caused some ATAPI drives to
turn off DMA for REQ_TYPE_BLOCK_PC commands while burning and thus degrading
performance and ultimately causing an excessive amount of underruns.

The issue is documented also in:
http://bugzilla.kernel.org/show_bug.cgi?id=11742

.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: Valerio Passini <valerio.passini@unicam.it>
[bart: fixup patch description per comments from Sergei Shtylyov]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>