scsi: fix compiler warning with DEBUG and 48bit LBAs

scsi: fix compiler warning with DEBUG and 48bit LBAs

Commit 2b42c9317db ("ahci: support LBA48 data reads for 2+TB drives")
introduced conditional code which triggers a warning when compiled
with DEBUG enabled:

In file included from common/cmd_scsi.c:12:0:
common/cmd_scsi.c: In function 'scsi_read':
include/common.h:109:4: warning: 'smallblks' may be used uninitialized in this function [-Wmaybe-uninitialized]
...

Since this is for debug only, take the easy way and initialize the
variable explicitly on declaration to avoid the warning.
(Fix a nearby whitespace error on the way.)

Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Andre Przywara <osp@andrep.de>