scsi: sd: Improve sd_print_capacity()

There is no need to call twice string_get_size() when the capacity messages
are not going to be printed. Reverse the message output condition to return
early and avoid executing string_get_size() when it is not necessary.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>