bsg: use pr_debug instead of hand crafted macros

Use pr_debug instead of hand crafted macros. This way it is not needed to
re-compile the kernel to enable bsg debug outputs and it's possible to
selectively enable specific prints.

Cc: Joe Perches <joe@perches.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>