ata: libata-core: remove redundant err_mask variable

Return value from ata_exec_internal() directly instead of
taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>