tty: serial: sh-sci: set error code when kasprintf fails
When the call to kasprintf() returns a NULL pointer, function
sci_request_irq() frees the preallocated memory and returns 0 is
returned. Because 0 means no error, the caller of sci_request_irq()
will keep going, and the freed memory may be used or freed again. To
avoid the above issue, this patch assigns "-ENOMEM" to the return
variable ret.