ata: libata-scsi: simplify __ata_scsi_queuecmd()

This patch cleans up the code of __ata_scsi_queuecmd(). Since each
branch of the "if" condition check that scmd->cmd_len is not zero, move
this check out of the "if" to simplify the conditions being checked in
the "else" branch.

While at it, avoid the if-else-if-else structure using if-else if
structure and remove the redundant rc local variable.

This patch does not change the function logic.

Signed-off-by: Wenchao Hao <haowenchao@huawei.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>