scsi: isci: avoid array subscript warning

I'm getting a new warning with gcc-7:

isci/remote_node_context.c: In function 'sci_remote_node_context_destruct':
isci/remote_node_context.c:69:16: error: array subscript is above array bounds [-Werror=array-bounds]

This is odd, since we clearly cover all values for enum
scis_sds_remote_node_context_states here. Anyway, checking for an array
overflow can't harm and it makes the warning go away.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>