ide: Fix error messages from static code analysis (no real error)

Report from smatch:
hw/ide/core.c:1472 ide_exec_cmd(423) error: buffer overflow 'smart_attributes' 8 <= 29
hw/ide/core.c:1474 ide_exec_cmd(425) error: buffer overflow 'smart_attributes' 8 <= 29
hw/ide/core.c:1475 ide_exec_cmd(426) error: buffer overflow 'smart_attributes' 8 <= 29
...

The upper limit of 30 was never reached because both for loops terminated
when 'smart_attributes' reached end of list, so there was no real buffer
overflow.

Nevertheless, changing the code not only fixes the error report, but also
reduces the size of smart_attributes and simplifies the for loops.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
