[SCSI] sd: retry read_capacity on UNIT_ATTENTION

Hazard testing uncovered yet another bug in sd. Under heavy reset
activity the retry counter might be exhausted and the command will be
returned with sense UNIT_ATTENTION/0x29/00 (POWER ON, RESET, OR BUS
DEVICE RESET OCCURRED). In those cases we should just increase the
retry counter again, retrying one more to clear up this Unit Attention
state.

[jejb: update to work with RC16 devices and not to loop endlessly]
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>