ide-cd: merge cdrom_do_packet_command() and cdrom_do_block_pc()

Add REQ_TYPE_{SENSE,ATA_PC} requests handling to cdrom_do_block_pc()
and remove cdrom_do_packet_command().

There should be no functionality changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>