scsi: sd_zbc: Remove superfluous assignments

A value is assigned to the variable 'capacity' in sd_zbc_read_zones()
but that value is never used. Hence remove the variable 'capacity'.

[Damien: There is no need to initialize to 0 the variable 'ret'
in sd_zbc_read_zones()]

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>